Conventionally, there is a known rotary damper which gives a predetermined braking force to a subject to be controlled which is rotated, thereby moderating its rotational motion.
The rotary damper includes a vane disposed in a fluid chamber in which viscous fluid is charged. The rotary damper generates a resistance against the viscous fluid by rocking the vane. There are a one-way rotary damper in which a check valve is provided so that the braking force can be exhibited only when the vane rocks in one direction (e.g., see the following patent documents 1 and 2), and a two-way rotary damper in which no check valve is provided so that the braking force can be exhibited irrespective of the rocking direction of the vane.
In this kind of rotary damper, the vane rocks and viscous fluid is pressed, and a resistance is generated when the viscous fluid moves through a small gap between the vane and a casing, and the resistance moderates the rotational motion of the subject to be controlled.
Therefore, the magnitude of the braking force exhibited by the rotary damper can be changed by changing a size of a gap or the like through which the viscous fluid passes when the viscous fluid moves. That is, if the gap is increased in size, the resistance of the viscous fluid is reduced and thus, the braking force can be reduced. If the gap is reduced in size on the contrary, the resistance of the viscous fluid is increased and thus, the braking force can be increased.
In the conventional rotary damper, the size of the gap through which the viscous fluid passes when the viscous fluid moves is usually constant. Thus, the exhibited braking force is also constant.
In a rotary damper in which the exhibited braking force is constant, when a load is small, the braking force becomes large relatively and when the load is great, the braking force becomes small relatively. Therefore, when the load is varied, the rotation speed of the subject to be controlled is largely varied.
Therefore, if such a rotary damper is applied to the subject to be controlled which has an accommodating section for accommodating an article such as an inner lid of a console box of an automobile or a glove box disposed in an opening formed in an instrument panel of an automobile, and in which the accommodating section is turned, a rotational moment of the subject to be controlled is small when no article is accommodated, and since a load applied to the rotary damper is small, the rotational motion of the subject to be controlled becomes extremely slow. On the contrary, when an article is accommodated, the rotational moment of the subject to be controlled is great and the load applied to the rotary damper becomes great and thus, the rotational motion of the subject to be controlled adversely becomes fast.
There is also a known rotary damper in which a size of a gap or the like through which viscous fluid passes when the viscous fluid moves is changed by operating the gap from outside, and the exhibited braking force can be adjusted (e.g., see the following patent documents 3 and 4).
In such a rotary damper, however, although the braking force can be adjusted, this adjustment is carried out based on a premise that a load to be applied to the rotary damper is constant after the adjustment. Thus, even if the braking force exhibited in accordance with a subject to be controlled is adjusted at initial stage of installation of the rotary damper, if a weight of the subject to be controlled is changed thereafter and a load to be applied to the rotary damper is changed, it is not possible to rotate the subject to be controlled at desired rotation speed unless the braking force is again adjusted.
Further, such a rotary damper must be operated from outside to adjust the braking force. Thus, if the rotational moment of the subject to be controlled is frequently changed and its changing amount is not constant like the inner lid of the console box or the glove box, this rotary damper is not suitable. That is, if the rotary damper is applied to such a subject to be controlled, whenever the rotational moment is changed as an article is loaded and unloaded, the braking force of the rotary damper must be adjusted again by predicting the changing amount of the rotational moment and operating the rotary damper from outside. Thus, it is difficult to appropriately adjust the braking force, and its operation is extremely troublesome and inconvenient.
In the conventional one-way rotary damper, a valve which realizes the one way rotary damper is formed as an independent member and then, the valve is assembled as one constituent part of the rotary damper. Thus, the number of parts is increased, a procedure for assembling the valve is necessary, and this increases the producing cost.
The rotary damper can moderate the rotational motion of the subject to be controlled by its shock absorbing effect. Therefore, when the rotary damper is applied to a reclining seat of an automobile, it is possible to moderate the forward rotational motion of a seat back against a biasing force of a spring member of a reclining mechanism which biases the seat back of the seat forward (see the following patent document 5 for example).
In the conventional rotary damper, however, the braking force can not be adjusted in accordance with the change in load. Therefore, in a reclining seat from which a head rest can be detached, the rotational moment of the seat back is changed between a case in which the head rest is attached and a case in which the head rest is detached. Thus, the rotation speed of the seat back is largely changed depending upon presence and absence of the head rest.
As other auto part, it is proposed to use the rotary damper also for an arm rest (see the following patent document 6 for example). However, in the arm rest having an accommodating section for articles, the rotational moment of the arm rest is changed depending upon a case in which the article is accommodated and a case in which no article is accommodated. Thus, in a rotary damper which can not adjust the braking force in accordance with the change in load, the rotational moment of the arm rest is changed, and its rotation speed is largely changed.
As a rotational motion assistant mechanism having a spring member which biases a subject to be controlled in one direction, there is a known mechanism which can adjust a biasing force of a spring member applied to the subject to be controlled by utilizing a fact that a stress of the spring member is changed by changing a position of a fulcrum of the spring member (see the following patent document 7 for example).
According to such a rotational motion assistant mechanism, however, since the biasing force of the spring member applied to the subject to be controlled is adjusted, a user must somehow operate the mechanism to change the position of the fulcrum of the spring member, and such an operation is troublesome and inconvenient.
The followings are conventional arts related the present invention:                Patent Document 1: Japanese Patent Application Laid-open No. H7-301272        Patent Document 2: Japanese Patent Application Laid-open No. 2002-81482        Patent Document 3: Japanese Patent Application Laid-open No. H7-197970        Patent Document 4: Japanese Patent Application Laid-open No. H7-301272        Patent Document 5: Japanese Patent Application Laid-open No. H8-38290        Patent Document 6: Japanese Patent Application Laid-open No. 2002-67767        Patent Document 7: Japanese Patent Application Laid-open No. 2001-169840        
The present invention has been accomplished in view of the above-described circumstances, and it is an object of the invention to provide a rotary damper capable of automatically adjusting a braking force exhibited in correspondence with change in load. It is another object of the invention to provide an auto part in which variation in rotation speed is small even if the rotational moment is changed. It is another object of the invention to provide a rotational motion assistant mechanism capable of automatically adjusting a biasing force of a spring member applied to a subject to be controlled in correspondence with change in rotation moment of the subject to be controlled.